Refinement of structure for a solvate-free crystal of the bromo-1-n-butylimidazole-iron(III) derivative of the "picket fence" porphyrin (meso-tetra(alpha, alpha, alpha, alpha-o-pivalamidophenyl)porphine, H2 (TpivPP)), (Br)Fe(BuIm) (TpivPP), should provide a significantly more quantitative description of structure than those reported for solvated crystals of the dioxygen-1-methylimidazoleiron(II) and carbonyl-1-methylimidazoleiron (II) derivatives, (O2) Fe (1-MeIm)(TpivPP) and (OC)Fe(1-MeIm)(TpivPP), respectively. (cf., J.P. Collman et al., J. Amer. Chem. Soc., 97, 1427 (1935); J.L. Hoard in "Porphyrins and Metalloporphyrins," K.M. Smith, Ed., Elsevier Scientific Publishing Company, Amsterdam, 1975, pp. 366-368). Structure determination is in prospect for a rather good crystal of empirical composition, Fe(TPP) 2THF, wherein the (TPP) ligand is derived from meso-tetraphenylporphine, H2(TPP), and THF denotes tetrahydrofuran. Because the magnetic susceptibility corresponds to high-spin (S equals 2) iron(II), we expect to find one half of the tetrahydrofuran coordinated to the metal atoms in five-coordinate (THF)Fe(TPP) molecules and the other half to appear as solvate in the crystal. Prompted by Rougee and Brault's detailed study (Biochemistry, 14:4100 (1975)) of the complexing equilibria in benzene solution of deuteroheme with carbon monoxide and a judicious selection of other liganding species, we propose to prepare a number of carbonyliron(II) derivatives of meso-tetraphenylporphine and to determine structure for those which afford crystals that are usable for X-ray analysis.